This invention relates to a method for treating diene rubber to improve the properties of the rubber, and to rubber compositions having improved green strength or plasticity properties, or lower hysteresis when cured.
The introduction of synthetic diene rubber as a total or partial replacement for natural rubber in portions of pneumatic tires and other rubber articles presented problems in that the properties of the synthetic materials differed from that of natural rubber. Significant areas of difference concerned green strength and plasticity. Typically, unvulcanized synthetic diene rubber has significantly lower green strength and higher plasticity than does natural rubber. This difference has presented problems; for example, articles such as tires failed to maintain their structural integrity during handling and shaping prior to vulcanization. As a result, searching for improvements in the green strength and plasticity of rubber compounds containing synthetic rubber has been a continuing effort.
Synthetic polyisoprene has been produced which resembles natural rubber closely, consisting of essentially all cis-1,4-polyisoprene. Perhaps the most significant difference between synthetic polyisoprene and natural rubber is that the former has considerably less green strength. Thus, much of the effort toward green strength improvement concerns synthetic polyisoprene. Other synthetic diene rubbers which are widely used are polybutadiene and styrene-butadiene copolymer rubber (SBR).
Hysteresis in a cured (vulcanized) rubber compound is directly related to heat build-up. High-hysteresis rubber, on repeated flexing, rapidly heats up. In certain applications, such as in the treads of truck tires, the degree of heat build-up in use can be such that thick sections in the treads are degraded to the point of failure, resulting in separations and delaminations, often to the extent that the entire tread peels off the tire. The synthetic diene rubber in the tire normally exhibits higher hysteresis than natural rubber, so that even in blends with natural rubber the synthetic diene rubber gives compositions having a higher heat buildup than all-natural rubber compositions.
Efforts to improve the properties of diene rubber include the incorporation of nitrosoanilinoalkane compounds therein, as shown in U.S. Pat. Nos. 3,151,161 and 3,225,100. Problems of dispersion of these materials into rubber have been encountered, made more acute by the fact that relatively small amounts of them are generally used.